Liquid level change detector



Feb. 1, 1938. OBERMAIER 2,106,756

LIQUID LEVEL CHANGE DETECTOR Original Filed Oct. 5, 1935 V FREON CONDENSER iiliiiliiliiiliiliiiliiliT'il'ill I Hll i FLU/D LEVEL OUTLET -fim John Fl. UbEPmaiEP BY W HZZUPTLEY Patented Feb. 1, 1938 LIQUID LEVEL CHANGE DETECTOR John A. Obermaier, Chicago, Ill.

Original application October 3, 1935, Serial No. 43,394. Divided and this application April 13,

1936, Serial'No. 74,013

7 Claims.

This application is a division of my pending application, Serial No. 43,394, filed October 3, 1935, and relates to apparatus and method for detecting changes in liquid level, and more particularly to such apparatus and method as are adapted for use with mechanical refrigerators for actuating an alarm to indicate a loss of refrigerant.

Consider, by way of example, a mechanical refrigerator using dichloro-difluro methane (CClzFz), hereafter called Freon, as the refrigerant. In the event of leakage of the gas from the system it is desirable to have means for actuating an alarm when a predetermined amount of gas has been lost. In accordance with the teachings of the present invention this result is. accomplished by providing means responsive to a drop in liquid levelof the Freon, if the drop in level persists for a predetermined length of time, for actuating an alarm. It is necessary to provide the time delay because during each starting operation of the compressor the liquid level normally drops below the level at which, if prevalent during non-operating periods, the alarm should be'actuated. The time lag should be about four minutes, although this may vary in different installations. The alarm is actuated only if the liquid level drop persists for more than the predetermined time lag period.

It is a further object of the present invention to provide an improved liquid level change responsive device which will be simple and reliable in its operation, which will have substantially no moving parts and thus not be subject to wear and tear, and which may be designed to have the requisite time lag in operation, and wherein the time lag is not likely to change with continued usage of the device.

'It is a further object of the present invention to provide means that detects a change in liquid level by detecting differences in heat conductivity of the ambient medium.

It is a still further object of the present invention to provide a liquid level change detecting means which is electrical in operation and in which the time lag may be changed in a simple manner, as by altering the amount of external resistance in the c1rcuit.

It is a still further object of the present invention to provide a liquid level change detecting device with means for indicating, at all times, if it is in normal operative condition, so that nonoperation of the device is a positive indication that the liquid level is on one side or the other of its critical position, without the uncertainty as to whether or not the non-operation is due I safety bulb embodying my invention;

Figure 3 is a diagram of the electric circuit connections; and 15 Figures 4 and 5 are diagrammatic views illustrating modifications of my invention.

Reference may now be had more particularly to Figure 1 of the drawing.

In this figure I have shown at l a condenser of any standard type of refrigerating apparatus, the refrigerant in this case being Freon. The condensed refrigerant flows into a container 2, and thence by way of a pipe 3 to the evaporator, in a well known manner. The apparatus is of the intermittently operated type, and normally the liquid level in the container is as indicated by the dotted line 4, when the condenser is not in operation. On starting of the condenser the liquid level drops appreciably, for a short interval of time. In the normal operation of the system, when the mechanism stops, the liquid level returns to the point indicated at 4.

A safety bulb, indicated at 5, is mounted with its end projecting into the container 2, below the normal liquid level 4. Should there be a loss of refrigerant, the normal liquid level '4 will drop. If the loss is so great that the normal liquid level drops below the level of the safety bulb 5 it is desirable that an alarm be actuated to indicate such loss of refrigerant. The safety bulb of the present invention effects such a result, as will be presently set forth. During normal starting periods-of the condenser the liquid level may drop below the level of the safety bulb, and remain below that level for short intervals. It is desirable that the time lag in operation of the safety bulb shall be greater than the time interval during which the liquid level remains below the level of the bulb in the normal operation of the apparatus, but that the safety bulb should function only if the drop in liquid level persists for a greater length of time, thus indicating a permanent loss of refrigerant.

Reference may now be had more particularly to Figure 2 showing the construction of the safety bulb. The safety bulb includes a tubular metal casing 8 closed at one end and having a head 9 at the open end thereof for facilitating screwing of the bulb into the container 2. Within the metal casing there is mounted a unit comprising a thermostatic switch l and an electric heater H which are enclosed in a copper tube 12 that fits snugly within the metal casing. The metal 8 is preferably iron, or steel, and of an appreciably lower heat conductivity than that of the copper tube E2, the tube l2 being selected because of its high heat conductivity. The thermostatic switch to is connected in parallel with the resistance heater H, being normally open and adapted to shunt out the heater when the temperature within the tube l2 exceeds'that for which the thermostat is set. The thermostatic switch is mounted on a block of insulation l4 over which the end of the tube 12 fits snugly, and the resistance heater l i, which is wound on a hollow open ended spool, also fits snugly within the copper tube, whereby the unit constitutes a simple assembly.

In Figure 3 there is shown the circuit connections for the safety bulb of Figure 2. The heater H and thermostatic switch ID are connected in circuit with an indicating lamp I6 and an external resistance l1, and alarm l8, to a source of alternating or direct current at 20. .During normal operation of the system, when the switch 10 is open, a small current flows through the circuit including the resistance II, and lamp ii, that current being suflicient to cause the lamp IE to glow with a dull red light. The current flowing through the resistance [1 and alarm 18 is insuflicient to actuate the alarm. If the switch 10 is closed, the current flowing through the circuit is thereby increased, so that the lamp I8 burns brightly, thus giving a visible signal indicative of the closure of the switch l0, and the current flowing through the resistance I! and alarm I8 is sufficient to actuate the alarm.

An explanation will now be given of the operation of the system of Figure 1. During normal operation of the condenser unit heat is being supplied to the heater H at a. small fixed rate, say approximately 50 watts. The heat thus generated is rapidly conducted by the copper tube [2 and then conducted outwardly by the metal casing 8. The temperature within the copper tube,

' which is the temperature that controls the thermostatic switch l0, thus reaches a value which is determined by the rate of heat conduction from the surface of the casing 8, which in turn is controlled by the heat conductivgy of the ambient medium about the metal casing. when the liquid level in the container 2 is above the safety bulb, as indicated at 4, this ambient medium is liquid Freon, which has a comparatively high heat conductivity. When the liquid level within the container 2 drops below the safety bulb, the ambient medium is Freon gas. which has a. comparatively lower heat conductivity. As a result, when the liquid level is below the safety bulb, the temperature of the safety bulb must rise to a higher value in order that it may continue to dissipate the substantially fixed heat input, namely, 50 watts. The rise in temperature causes the thermostatic switch ill to close. The mass of the metal of the bulb 8 is so correlated to the rate of heat input that it takes a predetermined period of time for the mass to rise to the temperature required for operation of the switch l0, say four or live min utes. This time interval is longer than the time during which the liquid level 4 drops below the level of the bulb in the normal starting operations of the condenser. Therefore the switch I 0 will not close due to a drop in liquid level in the normal operation of the condenser, but it will close when the liquid level 4 permanently drops, as due to a loss of refrigerant.

If the value of the resistance ll is changed, the normal heat input to the heater H will change, thus changing the time required to heat the metal casing 8, and changing the time lag for the operation of the thermmtatic switch H). The construction is made such that in the operation of the safety bulb the switch in, once closed, remains closed for, say, three or four minutes, whereupon there is sufficient cooling of the bulb to cause the switch ID to open and reestablish the circuit through the heater l l and cause reclosure of the switch I 0 three or four minutes thereafter.

Reference may now be had more particularly to Figure 4 wherein I have shown a somewhat different embodiment of my invention. Insofar as the parts illustrated in Figure 4 are similar to those previously described, similar reference numerals have been used. The liquid container, forming a part of the refrigerant circuit, is indicated at 2. Within this container are mounted two safety bulbs, indicated at 5a and 5b, of identical construction. These bulbs comprise a metal casing similar to the casing 8, and have a resistance heater on the inside thereof similar to the heater H, but do not have a thermostatic switch. A copper tube 25, corresponding to the tube I2, is also provided for conducting the heat from the resistance heater to the wall of the metal casing of the bulb. In this construction the two heater elements are of wire having a comparatively high current flowing through the upper winding of the diiferential relay 28 is the same as inthe lower winding, and no energization of the relay results. The current flowing through the heater element of the two safety bulbs results in a fixed heat input into the two bulbs, say 50 watts, which heat is dissipated through the copper tube and the outer metal casing to the medium surrounding the bulbs, namely, the liquid Freon. The temperature rise on the interior of the bulbs, therefore, is determined by the heat conductivity of the liquid Freon. If the fluid level in the container 2 drops below the bulb 5a, but not below the bulb 5b, the temperature of the bulb 5b remains as before, but the temperature of the bulb 5a rises because the heat conductivity of the surrounding medium, namely, gaseous Freon, is appreciably less than that of liquid Freon. The rate of temperature rise is determined by the mass of the metal casing 8, as was described in connection with Figure 2. When the temperature within the safety bulb 5a reaches a predetermined value, approximately the equilibrium temperature, the change in resistance of the heater in the bulb 5a is suflicient to eifect an appreciable reduction in the current flowing therethrough and through the upper winding of the relay 28, so that the relay 28 operates to close its contact and establish a circuit for a visual signal l6 and the audible signal I 8.

In Figure 5 I have shown the safety bulbs 5a and 5b, of a structure the same as in Figure 4, connected to constitute two arms of a Wheatstone bridge, the other two arms being indicated at 30 and 31. During normal operation of the system of Figure the bridge is substantially balanced so that the current flowing through the lamp l6 and the alarm i8 is insufiicient to actuate the alarm.

' If desired the bridge may be adjusted to a sumcient current flows to actuate the alarm l8 and to cause the visible signal Hi to lightup brightly.

An adjustable resistance II' may, if desired, be provided. By adjusting this resistance it is possible to change the amount of current that flows through the heaters 5a and 5b during normal operation of the system and thus effect a change in ,the time lag'oi operation of the bulb 5a. A similar resistance may also be provided in Figure 4.

In compliance with the requirements of the struction here shown, the same being merely illustrativeof the invention.

What I consider new and desire to secure by Letters Patent is: V v

1. Means for detecting a change in, liquid level comprising a hollow mass of material at the level where changes are to be detected, an electric resistance heater in said mass, means for electrically heating the resistance to supply heat to the mass at a substantially fixed rate, whereby the temperature rise of the mass is determined by the heat conductivity of theambient medium, and means for detecting a temperature change in the mass including means responsive to pronounced temperature changes of the mass resulting from a change in heat conductivity of the ambient medium as thecliquid level changes an amount to change the character of the medium surrounding the mass, the amount and kinds of material in the mass being correlated with the rate at i which heat is introduced therein by the heater and dissipated therefrom so as to produce a lag exceeding one minute in the time required for equilibrium temperature conditions to be attained following a change in liquid level such as results in a change in the ambient medium around the mass.

2. Means for detecting a change in liquid level comprising a hollow'mass of material at the level where changes are to be detected, an electric resistance heater in said mass, means fo r electrically heating the resistance to supply heat to the mass at a substantially fixed rate, whereby the temperature'rise of the mass is determined by the heat conductivity of the ambient medium, and means for detecting a temperature change in the mass including means responsive to pronounced temperature changes of the mass resulting from a change in heat conductivity of the ambient medium as the liquid level changes an amount to change the characterof the medium surrounding the mass, the amount and kinds of material in the mass being correlated with the rate at which heat is introduced therein by the heater and dissipated therefrom so as to produce a lag exceeding one minutein the time required for equilibrium temperature conditions to be attained following a change in liquid level such as results in a change in the ambient medium around the mass, and means for changing the time lag comprising means located outside of the mass for altering the rate of heat supplied by the heater.

3. Means for detecting a change in liquid level comprising a hollowmass of material mounted in a fixed position at the level where changes are to be detected, an electric resistance heater in said mass, a copper tube surrounding the heater and fitting snugly within said mass for conducting heat from the heater to said mass and projecting beyond the heater lengthwise of the mass for a distance at least equal to the length of the heater, Said mass having .a substantially lower heat conductivitythan that of the tube, means for electrically heating the resistance to supply heat to the mass at a substantially fixed rate, whereby the temperature rise of the mass is determined by the heat conductivity of the ambient medium, and means for detecting a temperature change in the mass including means responsivev to pronounced temperature changes of the mass resulting from a change in heat conductivity of the ambient medium as the liquid level changes an amount to changethe character of the medium surrounding the mass, the amount and kinds of material in the mass being correlated with the rate at which heat is introduced therein by theheater and dissipated therefrom so as to produce a lag exceeding one minute in the time required for equilibrium ternperature conditions to be attained following a change in liquid level such as results in a change in the ambient medium around the mass;

, 4. Means for detecting a change in liquid level comprising a bulb of heat conducting material at the level where changes are to be detected, whereby a change in liquid level to be detected will result in a change in the ambient medium about the bulb, an electric heater in the bulb and so located therein that substantially all of the heat generated thereby travels through the wall of the bulb to the portion thereof that is submerged in the liquid when the liquid level is above that of the bulb, means for supplying electrical energy to the heater at a substantially fixed rate; whereby the temperature rise of the bulb above that of the surrounding medium is determined by the heat conductivity of the surrounding medium, and is of a predetermined value when the liquid level is through the heater to effect a substantial reduction in current flowing through the same responsive to the rise in temperature to lower the tem\- perature of the bulb below that required for operation of the switch, whereby periodically repeated opening and closing of the switch results, and alarm means in the circuit and operated responsive to a change effected by operation of the switch, whereby the alarm means is actuated periodically as controlled by the switch the amount and kinds of material in the mass to be heated being correlated with the rate of heat generation by the heater so as to produce a time lag exceeding one minute in the operation of the switch on said change in liquid'level.

5. Apparatus of the class described comprising a tube of material of high heat conductivity, an electric heating element at one end thereof, .a thermal responsive switch at the otherend and heated by said element, and a second tube surrounding the first tube and of a material having a substantially lower heat conductivity than the first tube, the heat generated by the heater being dissipated through the first tube to the second and thence to the ambient medium, and thermal switch controlled circuit connections within one of said tubes for controlling the treating element.

6. Means for indicating a change in liquid level comprising a hollow mass of material in the liquid at the level where changes are to be detected, an

electric heater in said mass, 2. thermal responsive switch in said mass and heated by said heater, heat conducting means in said mass and connecting the switch with the heater, and means for supplying current to the heater of a substantially fixed amount, the current supplied being of such a value that the rate of heat generation in the mass is so correlated with the heat radiating surface of the mass that the switch is operated by the temperature change of the mass upon a change in the heatuconductivity of the medium surrounding the mass as the liquid level changes sufficiently to change the exposure to liquid of at least a part of the mass.

7. Means for indicating a change in liquid level comprising a hollow 'mass of material in the liquid at the level where changes are to be detected, an electric heater in said mass, a thermal responsive switch in said mass and heated by said heater, and means for supplying current to the heater of a substantially fixed amount, the current supplied being of such a value that the rate of heat generation in the mass is so correlated with the heat radiating surface of the mass that the switch is operated by the temperature change of the mass upon a change in the heat conductivity of the medium surrounding the mass as the liquid level changes sumciently to change the exposure to liquid of at least a part of the mass,

and the amount and kinds of material in the mass to be heated being correlated with the rate of heat generation by the heater so as to produce a time lag exceeding one minute in the operation of the switch upon said change in liquid level.

JOHN A. OBERMAIER. 

